1. Area of the Art
The present invention relates to the field of manufacture of semiconductor wafers. More particularly, the present invention relates to the field of detecting transition metal impurities in the manufacturing process of semiconductor wafers.
2. Description of the Prior Art
Transition metal impurities, even at relatively low concentrations, affect device yields, performance and reliability. Metal contaminations can be introduced by process chemicals, materials, equipment and environments.
There has been a continuous effort to reduce the levels of metal contaminations and the latest requirements calls for a reduction of metallic impurity levels to be below surface concentrations of 10.sup.9 cm.sup.-2. It is necessary not only to monitor and purify process chemicals and materials, but also to verify the quantity of unwanted impurities that are contaminating the surface of the silicon wafers and the bulk of such wafers.
These requirements demand measurement methods of ever increasing sensitivity to reduce detection limits well below certain defined values (e.g., specified values, target values and the like). Unfortunately, many known measurement methods in this field are either slow, expensive, not sensitive enough, or have any combination of these shortcomings.
Currently there are two major groups of existing methods which have been used for analysis of surface contamination and near-surface region contamination of silicon wafers. The first group includes chemical analytical methods to determine total impurity concentrations. The second one consists of indirect measurement techniques dealing with dissolved and electrically active or optically active impurity fractions.
The first group of existing measurement methods involves direct detection of copper (Cu) on the silicon surface and near-surface region by analytical techniques. Examples of such direct detection methods include energy dispersed x-ray analysis (EDX) for detecting copper in a heavily contaminated silicon surface (useful when the number of copper atoms is a few percent of the number of silicon atoms), secondary ion mass spectrometry (SIMS) for detecting surface concentrations of about 10.sup.12 cm.sup.-2 of copper, Auger spectroscopy for detecting surface concentrations of about 10.sup.13 cm.sup.-2 of copper, total reflection x-ray fluorescence analysis (TRXF) or inductively coupled plasma mass spectroscopy (ICP-MS) for detecting surface concentrations of about 10.sup.10 cm.sup.-2 of copper.
The second group of existing measurement methods involves indirect detection of copper surface contamination via electrical or optical characterization of copper in the bulk silicon after high-temperature annealing. Examples of such indirect detection methods utilizing electrical analysis include deep level transient spectroscopy (DLTS) and surface photo voltage (SPV). Examples of such indirect detection methods utilizing optical analysis include photo-luminescence (PL) analysis of the copper-related defects. In order to evaluate the surface contamination, the copper impurity must first be driven into the wafer bulk. This can be done either by conventional furnace annealing followed by sample quenching, or by rapid thermal annealing. The bulk concentration of copper-related defects is then re-calculated as a surface density.
The main disadvantage of the methods in the first group is the low sensitivity for detection of copper. Only total reflection x-ray fluorescence and inductively coupled plasma mass spectroscopy can be used for characterization of low surface contamination. However, these techniques cannot provide lateral analysis (resolution of the total reflection x-ray fluorescence is about 1 cm.sup.2) or in-depth analysis. The latter limitation is particularly important since copper is known to diffuse even at room temperature. In addition, the analysis of contamination by these techniques is time consuming, requires expensive equipment, and therefore is not suitable for the purpose of routine measurements.
The main disadvantage of the methods in the second group is that they require a high-temperature processing step. This can cause some significant errors in calculations of the copper concentrations. There are two fundamental sources for errors. First, high-temperature annealing can release copper impurities, which might have accumulated in the wafer during previous wafer/device processing. In this case, the surface contamination might be overestimated. Second, by being driven deep into the silicon bulk at high temperature, the copper impurity may be captured by gettering sites in the bulk or even at the back wafer surface. In epitaxial silicon wafers, copper may be effectively gettered in highly doped Czochralski silicon substrates containing a high density of crystal defects. In this case, the surface contamination appears to be significantly underestimated. Moreover, since copper strongly tends to precipitate during sample cooling, the cooling rate should be very high in order to detect copperrelated defects by deep level transient spectroscopy. However, even after a rapid quench, the fraction of electrically active copper is known to be of 10.sup.-4 to 10.sup.-2 of the total copper concentration.
Therefore, it is highly desirable to create methods for rapid and highly sensitive detection of transition metal contamination on the surface and in the subsurface region of single-crystal silicon wafers.